Solar energy can be used and stored by the efficient production of long-lived photo-induced charge separation--a state achieved in photosynthetic systems by the formation of a long-lived radical pair. A number of artificial systems have been reported that efficiently undergo photochemical charge transfer, unfortunately, the thermal back electron transfer often proceeds at an appreciable rate, limiting the utility of these systems. What is needed is a systems which has very efficient photoinduced charge transfer, and forms a charge-separated state which is long lived in air. The charge separation in these systems typically involves a redox reaction between a photoexcited donor and a suitable acceptor, resulting in the production of radical ion pairs illustrated by the formula: EQU D+hv.fwdarw.D* (1a) EQU D*+A.fwdarw.A.sup.- ( 1) EQU D.sup.+ +A.sup.- .fwdarw.D+A (2)
The cation and anion generated in this way are better oxidants and reductants, respectively, than either of the neutral groundstate molecules. To harvest the light put into this system, the oxidizing and reducing power of the photogenerated species must be used before the electrons are transferred back (equation 2) generating the starting materials. It is desirable to control this photochemically unproductive thermal fast back electron transfer reaction. One method has been to incorporate the donors and acceptors into solid matrices.
It is one objective of the present invention to provide compositions having efficient and sustained photoinduced charge separation states.